HUMAN ACTIVITY HAS dramatically shaped the natural environment with such wide-reaching effects that few believe natural spaces untouched by humans still exist. Building cities, extracting resources, travelling the globe, tilling the soil – all modify the planet in occasionally alarming ways.

Agriculture has become a large contributor to carbon emissions. In Canada, 10 percent of greenhouse gas emissions come from crop and livestock production, not including related activities to produce materials for agriculture such as fertilizer.

And despite technological advances and land-use changes to better contain fertilizer applied to farm fields, contamination of land and water systems by agricultural runoff remains a big problem for freshwater systems in North America.

Phosphorus is a big part of this problem. Most inorganic fertilizers contain phosphorus – it’s an important nutrient for crop growth, and is essential in modern agriculture. It can also find its way into ag-fields and waterways through livestock and human waste. As crops grow, they uptake phosphorus from the soil, though any phosphorus not used by the plant stays locked in the soil.

Figuring out how to grow crops while minimizing the ecological damage their fertilizers can cause has bedeviled researchers for decades.

A new understanding

For the first time, researchers have determined how to calculate the ‘tipping point’, a maximum amount of phosphorus, that a watershed can safely hold.

Led by Jean-Olivier Goyette, a doctoral student at the Université de Montréal, the research team looked at historical land-use data in Quebec to understand how phosphorus accumulates over time. By examining 23 watersheds near the St. Lawrence River basin in Quebec, Goyette estimated accumulated phosphorus levels over the past century and compared this data to phosphorus levels measured in the water over the past 26 years to determine when, exactly, local watersheds had overloaded.

“Think of the land as a sponge. After a while, sponges that absorb too much water will leak. In the case of phosphorus, the landscape absorbs it year after year after year, and after a while, its retention capacity is reduced.

The results, published in Nature Geoscience, show that watersheds retain an average of 2.1 tonnes of phosphorus per square kilometre before significant leaching into surface waters occurs. Goyette notes this “surprisingly low” threshold value is reached relatively quickly. By the 1950s, when mass agriculture began in Quebec, over half of the 23 watersheds had already surpassed their phosphorus limits.

“Think of the land as a sponge,” said Université de Montréal aquatic ecosystem ecologist Roxane Maranger. “After a while, sponges that absorb too much water will leak. In the case of phosphorus, the landscape absorbs it year after year after year, and after a while, its retention capacity is reduced. At that point historical phosphorus inputs contribute more to what reaches our water.”

The St. Lawrence basin is the second largest drainage basin in North America, and much of the land surrounding the river is intensively farmed. Based on their observations, the researchers estimate that an average agricultural watershed would surpass its phosphorus limit in as little as two to four years. And that eliminating the phosphorus already present in the basin could take hundreds, or even thousands of years if phosphorus inputs stopped immediately. Which they won’t.

Based on Goyette’s data, it’s likely that most intensively farmed areas around the world have already passed their tipping points.

The Future of Agriculture

The risk of surface water contamination by phosphorus has increased in recent years on farmed land. Between 1981 and 2011, the risk of surface water contamination by phosphorus rose by 50 percent in select Canadian locales, with fertilizer and higher livestock density accounting for most of the increase.

Removing phosphorus from agricultural practices is not a simple or even feasible solution, as crops need food to grow. And as the global population grows, there will be higher demand for food, water, land and other resources.

Meeting these demands while harmonizing with nature will be a challenge. Farmers must meet the needs of their crops while managing waste disposal and meeting environmental guidelines. Regular soil testing is needed to prevent over-application and leaching.

Yet the outlook isn’t entirely bleak. Goyette’s study can help guide governments, farmers and other researchers when approaching future farming techniques. Other studies are looking at novel ways to fertilize plants, including methods to extract existing, unused phosphorus from the soil. It’s already happening in some European countries, Goyette said. “Furthermore, phosphorus can be recycled and reused as fertilizer rather than accessing more of the raw mined material.”

Reducing phosphorus emissions from waste may also play a big role in reducing runoff, as will better methods for fertilizer application.

To ensure a sustainable future, these and other techniques are urgently needed to preserve the quality of water resources and the environment. It will require walking a fine line, but addressing these issues are crucial for the health of our freshwater ecosystems.